Structural Effect Analysis And Reasonable Completion State Correction Of Concrete Cable-stayed Bridge Based On Life Cycle

As the operation time goes on,the time-varying effect has more and more influence on the stress and deformation of cable-stayed bridge structure,the effects of shrinkage,creep and carbonization can not be ignored.On the basis of consulting a large number of literatures,it is found that most scholars only study shrinkage and creep or carbonation,and most of the researches on carbonization are focused on the material level.In contrast,there is little research on the influence of the both effects on the whole bridge structure in the whole life operation process.Taking a prestressed concrete cable-stayed bridge in Chongqing as the research object,this paper studied the concrete shrinkage and creep and carbonation in the time-varying effect,and analyzed the structural effect of the actual bridge in different stages of operation under the joint action of the effect,provides the basis for the operation and maintenance of the bridge.According to the research results of structural effect in operation period,the reasonable completed state of cable-stayed bridge based on time-varying effect of life cycle was modified.The main research contents are as follows:(1)Depended on the engineering background,the numerical analysis model was established by using the bridge finite element software Midas Civil.The minimum bending energy method was used to preliminarily determine the cable force of the completed bridge,and the cable force of the completed bridge was initially determined and optimized.The reasonable construction state was determined through the iterative analysis of "reverse demolition and normal installation" in the construction stage.The stress of the structure of the model is reasonable and the deformation is within the specification limit.It can provide model basis for further time-varying effect analysis.(2)Four shrinkage models at home and abroad,JTG 3362-2018,CEB-FIP 2010,ACI 209(92)and European standards were selected for parameter impact analysis.Under the same parameter conditions,the nominal shrinkage coefficient of ACI 209(92)mode decreases with the increase of the theoretical thickness,and its changing range is greater in the range of smaller thickness;The shrinkage strain calculated by ACI 209(92)is the fastest in the early stage,while the final value of CEB-FIP 2010 is the largest.(3)Four creep models at home and abroad,JTG 3362-2018,CEB-FIP 2010,ACI 209(92)and European standards were selected for parameter impact analysis.Under the same parameter conditions,the creep coefficient predicted by JTG 3362-2018 is the largest,and the nominal creep coefficient calculated by Eurocode decreases at the highest rate with the increase of relative humidity;When the concrete strength is small,the nominal creep coefficient calculated by CEB-FIP 2010 model is the most sensitive to the change of concrete strength.(4)Based on the finite element model,the influence of shrinkage and creep effect on the actual bridge structure under different codes and different operation time was analyzed.The deformation and internal force change of the main girder calculated in JTG 3362-2018 code is the largest among the four codes selected.There is little difference between the axial force and shear force of the main girder under different codes,but the maximum difference of bending moment is about 40%.The effects of shrinkage and creep on deflection,bending moment and stress of main girder of cable-stayed bridge are more obvious than those on cable force,shear force and axial force.(5)According to the measured data,the prediction formula of carbonation was modified,and the prediction formula of section damage and reinforcement performance degradation was derived.In different stages of structural deterioration,the section loss rate of reinforcement is different.When the concrete cover is corroded and cracked due to carbonation,the section loss rate of reinforcement increases greatly.The calculation results show that the time for carbonation depth to reach the surface of steel bar is about 45 years,the time for corrosion expansion cracking of protective layer is about 70 years,and the time to reach the maximum crack width is about 95 years.The steel section loss caused by carbonation is relatively slow in ordinary atmospheric environment,and only lost 0.9% when the carbonation depth reaches15 mm.(6)Based on the finite element model and the carbonation prediction formula,the structural effects of the actual bridge in different operation periods under the combined influence of concrete shrinkage and creep and carbonization were analyzed.The results show that the deflection and internal force of the main girder reach about 90% of that in 100 years when the shrinkage and creep affect 20 years.The change amplitude of internal force and deflection is very small in 30-70 years of operation.After 70 years of operation,carbonization leads to rust expansion and cracking of protective layer.Under the combined action of shrinkage creep and carbonization,the deflection of main girder in 100 years of operation changes by 44%compared with that in 80 years.The results show that the effect of shrinkage and creep is more significant in the early stage of operation,but it is not obvious in the middle stage of operation.When the carbonation depth reaches a certain degree,the internal force and deformation of the structure change significantly again.(7)According to the stress balance method,this paper proposed a reasonable completed bridge state correction method based on the safety of the whole life cycle of concrete cablestayed bridge,and modified the reasonable completed bridge state determined by the minimum bending energy method.The results show that under the modified completed bridge state,the mid span position and bending moment near the support are improved,and the structural safety and material utilization rate are improved.